Exposure warning circuit for photographic cameras

ABSTRACT

An exposure warning circuit for accurately warning whether a photograph to be taken will be over-exposed, properly exposed or under-exposed prior to photographing, comprising an exposure pulse generating circuit, reference pulse generating circuits, pulse-width comparison circuits and indicating means.

Umted States Patent 1 [111 3,792,648

Ishiguro Feb. 19, 1974 EXPOSURE WARNING CIRCUIT FOR 3,603,799 9/1971Nobusawa 95/10 ux PHOTOGRAPHIC CAMERAS [75] Inventor: Yasuo Ishiguro,Tokyo, [Japan Primary Examiner joseph E Peters, h [73] Assignee:Kabushiki Kaisha Koparu, Assistant Examiner-Michael L. Gellner T k Ja anAttorney, Agent, or FirmCushman, Darby and 221 Filed: Feb. 23, 1973Cushma 21 A l. N 430 1 PP O p 57 ABSTRACT [30] Foreign ApplicationPriority Data An exposure warning circuit for accurately warning Feb.25, 1972 Japan 47/19470 whether a photograph be taken will b v rexposed,properly exposed or under-exposed prior to [52] US. Cl. 95/10 CEphotographing, comprising an exposure pulse generat- [51] Int. Cl.- G03b7/00 ing circuit, reference pulse generating circuits, pulse- [5 8]Field of Search 95/10 CE; 250/214 P width comparison circuits andindicating means.

[56] Ree'ences 14 Claims, 3 Drawing Figures UNITED STATES PATENTS3,581,643 6/1971 Yoshimura 95/10 X '1 1 EXPOSIRE REFEREMJE REFERENCEPULSEA PULSE 5E GEPE T- GENEgfl- GEPERAT- ING CKT ING ING CKT (FOR OVER-(F'OR UNwR- EXPOSURE) EXPOSURE) Pmmeu tm' 3.792.648

SHEET 1 BF 2 POSU REFERENCE REFERENCE FEG CKT me ms CKT (FOR OVER- (FORUNEER- EXPOSURE) EXPOSURE) I u) A, 2 (REFERENCE PULSE) "J O F F v TIME(EXPOSURE PULSE) VOLTAGE TIME EXPOSURE WARNING CIRCUIT FOR PHOTOGRAPHICCAMERAS BACKGROUND OF THE INVENTION Field of the Invention The presentinvention relates to photographic cameras and more particularly, to anexposure warning circuit for photographic cameras.

DESCRIPTION OF THE PRIOR ART An exposure warning for photographiccameras has been given by directly measuring the resistance value o faphotoconductive cell, which is a light receiving element. Generally,however, the resistance value of a photoconductive cell changes inlogarithmic relation to the light intensity. Therefore, when theintensity of the incident light from the object to the photoconductivecell, i.e., when the brightness of the object to be photographed changesin a wide range, it is very difficult to measure the resistance value ofthe photoconductive cell over the whole range and consequently, theexposure warning is not given properly.

SUMMARY OF THE INVENTION dicating device according to the result ofcomparison.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a basic circuit diagramfor explaining the principle of the present invention;

FIG. 2 shows an explanatory diagram giving exposure pulse shapes andreference pulse shapes; and

FIG. 3 shows a circuit diagram of another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, showing abasic circuit for explaining the fundamental principle of the presentinvention, numeral 1 designates an exposure pulse generating circuit forgenerating an exposure pulse having a pulse width in inverse proportionto the intensity of the light from the object to be photographed.Numeral 2 designates a reference pulse generating circuit for generatinga reference pulse (b,) which has a predetermined pulse width forindicating that the exposure is excessive. Numeral 5 designates areference pulse generating circuit similar to the circuit 2 but forgenerating a reference pulse (b.,) for indicating that the exposure isinsufficient and the pulse width of the generated pulse is longer.

When inputs are given to bases of transistors T, and T in FIG. 1, thetransistor to which the input pulse (rising portion) is given earlierbecomes conductive. When one of transistors T, and T is in conductivestate, the

other transistor does not cause any change in its state even when aninput is given to it. For example, when an input with a pulse shape a,is given to the base of the transistor T, and an input with a pulseshape b, is given to the base of the transistor T as shown in FIG. 2,the transistor T, becomes conductive but the transistor T does notbecome conductive. That is, when no input is given to bases oftransistor T, nor T transistors T, and T and transistors T and T, areall non-conductive. When an input is given to the base of the transistorT, at first, the transistor T, becomes conductive. So, the transistor T2 also becomes conductive and the lamp L, for indicating that theexposure is excessive lights. Even when an input is given to the base ofthe transistor T after a certain time delay, the transistor T becomesonly ready for becoming conductive. At that time, however, the collectorvoltage of the transistor T is already decreased by lighting of the lampL,. So, the base voltage of the transistor T, is decreased, thetransistor T, becomes non-conductive and, consequently, the transistor Tcannot become conductive.

Therefore, the relation between the reference pulse and exposure pulseis as shown by the reference pulse shape b, and exposure pulse shape a,in FIG. 2, the over-exposure lamp L, lights to indicate that theexposing condition is an over-exposure. When said relation is as shownby the over-exposure reference pulse shape 1), and exposure pulse shapea of course the lamp L, does not light.

In FIG. 1, another pulse-width comparison circuit same as describedabove is composed of transistors T through T When the exposure pulseshape showing the intensity of the light from the object is as shown bythe pulse a, relative to the reference pulse shape b for under-exposuregenerated by the reference pulse generating circuit 5, transistors T andT become conductive, and the under-exposure warning lamp L lights toindicate that the exposure is insufficient.

In the above, cases of over-exposure and underexposure are describedaccording to the basic circuit. A more practical and concrete circuit isshown in FIG. 3 giving the same reference symbols to those parts havingfunctions same as those shown in FIG. 1 and FIG. 2. This circuit isarranged to give over-exposure warning, high-speed exposure indication,middle-speed exposure indication, low-speed exposure indication andunder-exposure warning by means of respective lamps L, through L lit bypulse-width comparison circuits arranged by transistors T, through T inthe same way as shown in FIG. 1. In this circuit, the exposure pulsegenerating circuit shown in FIG. 1 is composed of the block 1,,including transistors T through T,, and reference pulse generatingcircuits are composed of blocks 2,, and 5,, which comprise mono-stablemulti-vibrators. Besides, reference pulse generating circuits 3,, and4,,

for high-speed exposure and middle-speed exposure also comprisemono-stable multi-vibrators. Reference symbols T through T designatetransistors.

Reference symbol SW designates a selector switch used when aphotoconductive element Rx of the circuit according to the presentinvention is used in common to an exposure time control circuit for anelectric shutter. Reference symbol SW designates an ASA selector switchwhich is changed over among capacitors C, through C as required.Reference symbol SW, designates a selector switch for compensation ofexposure in case of EB photographing.

When the power switch SW, is closed in the state that when switches SWand SW are at positions shown in FIG. 3, reference pulse generatingcircuits 2,, through 5,, are actuated by the pulse voltagedifferentiated by the capacitor C, and resistor R,. At the same time,the capacitor C is charged through the photoconductive element Rx.Therefore, after a time in inverse proportion to the intensity of thelight from the object (i.e., the time equivalent to the exposure pulsewidth) after the switch SW, is closed, the transistor T becomesconductive. Consequently, the transistor T, becomes conductive, andtransistors T,, and T remain nonconductive. Thus, an input is given tothe base of the transistor T, as explained referring to FIG. 1. On theother hand, a reference pulse for over-exposure warning is applied tothe base of the transistor T by the mono-stable multi-vibratorcomprising transistors T and T Therefore, when the relation between thereference pulse and exposure pulse is as shown by the pulse shape b, andpulse shape a, in FIG. 2, Le, in case of over-exposure condition, thelamp L, lights.

in the above case, when widths of reference pulses for over-exposurewarning, high-speed exposure indication, middle-speed exposureindication and lowspeed exposure indication (used in common tounderexposure warning) are respectively a,, a a and 0,, they are ofcourse in the relation of a, a a a,,. Therefore, lamps L through L whichare not required to light, also tend to light. In the presentembodiment, this problem is solved as follows. That is, when the lamp L,lights, diodes D, through D, reduce base voltages of transistors T T,,,,T, and T (T is included here to prevent the lamp L from lighting afterthe lamp L, is put out) in order to prevent the other lamps L L,,, L,and L, from lighting when the over-exposure warning lamp L, is put on.When the exposure pulse width at a light intensity of the object is aand the relation of a, with reference pulse widths is a, a, 11 a,, lampsL L and L other than the highspeed exposure indication lamp L also tendto light. To put out those unnecessary lamps, diodes D through D areutilized in the same way as above. Regarding the other cases,unnecessary lamps do not light in the same way as described above.

In the above-mentioned embodiment, an exposure pulse having a pulsewidth equivalent to the exposure time in second is directly comparedwith the reference pulse. However, this method may be inconvenient inactual photographing because it will take a long time to give a warningwhen the light intensity from the object is low. To solve this problem,it is possible to condense the exposure pulse width (of course alsoreference pulse widths) as required. For example, capacities ofcapacitors C, through C, may be changed to make the exposure pulse widthabout l/500 times of the actual exposure time in second.

I claim:

1. An exposure warning circuit for photographic cameras comprising: atleast one exposure-warning indicating means, at least one pulse-widthcomparison circuit having two input terminals connected to saidexposure-warning indicating means, an exposure pulse generating circuitconnected to one of the input terminals of said pulse-width comparisoncircuit and generating an exposure pulse with a pulse width in inverseproportion to the light intensity of the object, and at least onereference pulse generating circuit connected to the other input terminalof said pulse width comparison circuit and generating a reference pulseof predetermined pulse width relative to the exposure time to be warned;said exposure warning circuit controls the operation of saidexposure-warning indicating means by making a comparison between pulsewidths of said exposure pulse and reference pulse.

2. An exposure warning circuit for photographic cameras according toclaim 1, in which said exposurewarning indicating means is a lamp.

3. An exposure warning circuit for photographic cameras according toclaim 1, in which said pulsewidth comparison circuit comprises a pair oftransistors respectively having a base connected to the collector of theother transistor in the pair and another pair of transistors havingcollectors respectively connected to respective emitters of aforesaidpair of transistors.

4. An exposure warning circuit for photographic cameras according toclaim 1, in which said exposure pulse generating circuit comprises aphotoconductive element and capacitors connectable in series to saidphotoconductive element.

5. An exposure warning circuit for photographic cameras according toclaim 1, in which said reference pulse generating circuit is arranged bya mono-stable multi-vibrator comprising transistors.

6. An exposure warning circuit for photographic cameras according toclaim 1, in which said reference pulse generating circuit is arranged togenerate a pulse of a width to warn over-exposure.

7. An exposure warning circuit for photographic cameras according toclaim 1, in which said reference pulse generating circuit is arranged togenerate a pulse of a width to indicate high-speed exposure.

8. An exposure warning circuit for photographic cameras according toclaim 1, in which said reference pulse generating circuit is arranged togenerate a pulse of a width to indicate middle-speed exposure.

9. An exposure warning circuit for photographic cameras according toclaim ll, in which said reference pulse generating circuit is arrangedto generate a pulse of a width to indicate low-speed exposure.

10. An exposure warning circuit for photographic cameras according toclaim ll, in which said reference pulse generating circuit is arrangedto generate a pulse of a width to warn under-exposure.

11. An exposure warning circuit for photographic cameras according toclaim 1, in which pulse widths of said exposure pulse and said referencepulses are respectively condensed to H500.

12. An exposure warning circuit for photographic cameras comprising: anover-exposure warning lamp, a first pulse-width comparison circuithaving two input terminals connected to said over-exposure warning lamp,a first reference pulse generating circuit connected to one of the inputterminals of said first pulsewidth comparison circuit and generating areference pulse for over-exposure warning, a high-speed exposureindicating lamp whose one terminal is connected to said over-exposurewarning lamp, a second pulsewidth comparison circuit having two inputterminals and having an output terminal connected to another terminal ofsaid high-speed exposure indicating lamp, a second reference pulsegenerating circuit connected to one of the input terminals of saidsecond pulse-width comparison circuit and generating a reference pulsefor high-speed exposure indication, an exposure pulse generating circuithaving an output terminal connected commonly to other input terminals ofsaid first and second pulse width comparison circuit and generating anexposure pulse of a width in inverse proportion to the light intensityof the object to be photographed, and a diode inserted between outputterminals of said first and second pulse-width comparison circuits, saidexposure warning circuit is arranged to light only one of saidover-exposure warning lamp and said high-speed exposure indicating lampby making a comparison between the pulse width of said exposure pulseand pulse widths of respective reference pulses.

13. An exposure warning circuit for photographic cameras according toclaim 12, in which said exposure warning circuit further comprises amiddle-speed exposure indicating lamp whose one terminal is connected tosaid high-speed exposure indicating lamp, a third pulse-width comparisoncircuit whose one input terminal is connected to the output terminal ofsaid exposure pulse generating circuit and whose output terminal isconnected to the another terminal of said middle-speed exposureindicating lamp, a third reference pulse generating circuit connected toanother terminal of said third pulse-width comparison circuit andgenerating a reference pulse for middle-speed exposure indication, adiode inserted between output terminals of said first and thirdpulse-width comparison circuits, and a diode inserted between outputterminals of said second and third pulse-width comparison circuit, saidexposure warning circuit is arranged to light only one out of said overexposure warning lamp, said high-speed exposure indicating lamp and saidmiddle-speed exposure indicating lamp by making comparison among pulsewidths of said exposure pulse and of reference pulses.

14. An exposure warning circuit for photographic cameras according toclaim 13, in which said exposure warning circuit further comprises alow-speed exposure indicating lamp and an under-exposure warning lamprespectively having a terminal connected to said mid dle-speed exposureindicating lamp, a fourth pulse width comparison circuit whose one inputterminal is connected to the output terminal of said exposure pulsegenerating circuit and whose one output terminal is connected to anotherterminal of said low-speed expo sure indicating lamp and whose anotheroutput terminal connected to another terminal of said underexposurewarning lamp, a fourth reference pulse generating circuit connected toanother input terminal of said fourth pulse-width comparison circuit andgenerat ing a reference pulse for low-speed exposure indicating pulse,diodes respectively inserted between output terminals of said first andfourth pulse-width comparison circuits, diodes respectively insertedbetween output terminals of said second and fourth pulse-widthcomparison circuits, and diodes respectively inserted between outputterminals of said third and fourth pulse width comparison circuits, saidexposure warning circuit is arranged to light only one lamp out of saidoverexposure warning lamp, said high-speed exposure indicating lamp,said middle-speed exposure indicating lamp, said low-speed exposureindicating lamp and said under-exposure warning lamp by makingcomparison between pulse widths of said exposure pulse and of eachreference pulse.

1. An exposure warning circuit for photographic cameras comprising: atleast one exposure-warning indicating means, at least one pulse-widthcomparison circuit having two input terminals connected to saidexposure-warning indicating means, an exposure pulse generating circuitconnected to one of the input terminals of said pulse-width comparisoncircuit and generating an exposure pulse with a pulse width in inverseproportion to the light intensity of the object, and at least onereference pulse generating circuit connected to the other input terminalof said pulse width comparison circuit and generating a reference pulseof pre-determined pulse width relative to the exposure time to bewarned; said exposure warning circuit controls the operation of saidexposure-warning indicating means by making a comparison between pulsewidths of said exposure pulse and reference pulse.
 2. An exposurewarning circuit for photographic cameras according to claim 1, in whichsaid exposure-warning indicating means is a lamp.
 3. An exposure warningcircuit for photographic cameras according to claim 1, in which saidpulse-width comparison circuit comprises a pair of transistorsrespectively having a base connected to the collector of the othertransistor in the pair and another pair of transistors having collectorsrespectively connected to respective emitters of aforesaid pair oftransistors.
 4. An exposure warning circuit for photographic camerasaccording to claim 1, in which said exposure pulse generating circuitcomprises a photoconductive element and capacitors connectable in seriesto said photoconductive element.
 5. An exposure warning circuit forphotographic cameras according to claim 1, in which said reference pulsegenerating circuit is arranged by a mono-stable multi-vibratorcomprising transistors.
 6. An exposure warning circuit for photographiccameras according to claim 1, in which said reference pulse generatingcircuit is arranged to generate a pulse of a width to warnover-exposure.
 7. An exposure warning circuit for photographic camerasaccording to claim 1, in which said reference pulse generating circuitis arranged to generate a pulse of a width to indicate high-speedexposure.
 8. An exposure warning circuit for photographic camerasaccording to claim 1, in which said reference pulse generating circuitis arranged to generate a pulse of a width to indicate middle-speedexposure.
 9. An exposure warning circuit for photographic camerasaccording to claim 1, in which said reference pulse generating circuitis arranged to generate a pulse of a width to indicate low-speedexposure.
 10. An exposure warning circuit for photographic camerasaccording to claim 1, in which said reference pulse generating circuitis arranged to generate a pulse of a width to warn under-exposure. 11.An exposure warning circuit for photographic cameras according to claim1, in which pulse widths of said exposure pulse and said reference pulseare respectively condensed to 1/500.
 12. An exposure warning circuit forphotographic cameras comprising: an over-exposure warning lamp, a firstpulse-width comparison circuit having two input terminals connected tosaid over-exposure warning lamp, a first reference pulse generatingcircuit connected to one of the input terminals of said firstpulse-width comparison circuit and generating a reference pulse forover-exposure warning, a high-speed exposure indicating lamp whose oneterminal is connected to said over-exposure warning lamp, a secondpulse-width comparison circuit having two input terminals and having anoutput terminal connected to another terminal of said high-speedexposure indicating lamp, a second reference pulse generating circuitconnected to one of the input terminals of said second pulse-widthcomparison circuit and generating a reference pulse for high-speedexposure indication, an exposure pulse generating circuit having anoutput terminal connected commonly to other input terminals of saidfirst and second pulse width comparison circuit and generating anexposure pulse of a width in inverse proportion to the light intensityof the object to be photographed, and a diode inserted between outputterminals of said first and second pulse-width comparison circuits, saidexposure warning circuit is arranged to light only one of saidover-exposure warning lamp and said high-speed exposure indicating lampby making a comparison between the pulse width of said exposure pulseand pulse widths of respective reference pulses.
 13. An exposure warningcircuit for photographic cameras according to claim 12, in which saidexposure warning circuit further comprises a middle-speed exposureindicating lamp whose one terminal is connected to said high-speedexposure indicating lamp, a third puLse-width comparison circuit whoseone input terminal is connected to the output terminal of said exposurepulse generating circuit and whose output terminal is connected to theanother terminal of said middle-speed exposure indicating lamp, a thirdreference pulse generating circuit connected to another terminal of saidthird pulse-width comparison circuit and generating a reference pulsefor middle-speed exposure indication, a diode inserted between outputterminals of said first and third pulse-width comparison circuits, and adiode inserted between output terminals of said second and thirdpulse-width comparison circuit, said exposure warning circuit isarranged to light only one out of said over exposure warning lamp, saidhigh-speed exposure indicating lamp and said middle-speed exposureindicating lamp by making comparison among pulse widths of said exposurepulse and of reference pulses.
 14. An exposure warning circuit forphotographic cameras according to claim 13, in which said exposurewarning circuit further comprises a low-speed exposure indicating lampand an under-exposure warning lamp respectively having a terminalconnected to said middle-speed exposure indicating lamp, a fourthpulse-width comparison circuit whose one input terminal is connected tothe output terminal of said exposure pulse generating circuit and whoseone output terminal is connected to another terminal of said low-speedexposure indicating lamp and whose another output terminal connected toanother terminal of said under-exposure warning lamp, a fourth referencepulse generating circuit connected to another input terminal of saidfourth pulse-width comparison circuit and generating a reference pulsefor low-speed exposure indicating pulse, diodes respectively insertedbetween output terminals of said first and fourth pulse-width comparisoncircuits, diodes respectively inserted between output terminals of saidsecond and fourth pulse-width comparison circuits, and diodesrespectively inserted between output terminals of said third and fourthpulse-width comparison circuits, said exposure warning circuit isarranged to light only one lamp out of said over-exposure warning lamp,said high-speed exposure indicating lamp, said middle-speed exposureindicating lamp, said low-speed exposure indicating lamp and saidunder-exposure warning lamp by making comparison between pulse widths ofsaid exposure pulse and of each reference pulse.